To understand why GitHub is worth so much, you have to be a programmer. For them, GitHub is a combination of LinkedIn, Twitter, and Google Docs: It's at once a place to showcase your skills, talk shop, store your work, and collaborate with others.

It's also profitable, from what Github cofounder and CTO Tom Preston-Werner told me recently. GitHub offers code repositories on its public website for free. It charges fees, starting at $7/mo., for private repositories; it also sells GitHub Enterprise, which lets businesses put code on their own servers, behind corporate firewalls.